The present invention relates to a clock generator which derives clock information from a digital bit stream for a digital demodulator.
According to known digital modulation techniques, such as eight-to-foufteen modulation (EFM), original eight bit codes of random clock spacings are converted to fourteen bit codes of the format having a minimum spacing of three clock pulses and a maximum spacing of eleven clock pulses to provide enough clock information to demodulate the EFM bit stream.
Conventional clock generators employed for demodulating such EFM bit streams include circuitry that detects a series of data bits having the minimum and maximum clock spacings and counts clock pulses generated by a voltage-controlled oscillator which are present during the minimum and maximum spacings of the detected data bits. Two count values are derived as a measure of the frequency of the clock pulse to control the frequency of the oscillator. One disadvantage of the prior art is that since the data bits of minimum and maximum clock spacings occur at random the phase-locked loop is likely to remain out-of-phase with the input bit stream for a substantial period if successive frames contain no data bits having mininum and maximum clock spacings. The aforesaid Co-pending application eliminates the problem by a closed loop phase and frequency control circuit that includes a voltage-controlled oscillator for generating clock pulses and a pulse generator for generating a window pulse in response to an input binary digit and supplies it to a phase comparator for comparison with the clock pulse. A frequency comparator is provided to detect whether the frequency of the clock pulse is below the lower limit of a predetermined range or above the upper limit of that range and produces a frequency control signal of different amplitudes depending on the relative value of the clock frequency to the lower and upper limits. The frequency control signal is combined with the output of the phase comparator and applied to the voltage-controlled oscillator.
However, it is desired to minimize disturbance which occurs in clock frequency if the input data bit stream is interrupted for a substantial period of time as in the case of a helical scan tape recorder wherein the tape is wound on a 90.degree. arc of a rotary drum and if the rate of the bit stream is varied abruptly as in the case of a digital tape recorder changing its tape transport speed.